<!DOCTYPE HTML>
<html>
    <head>
        <title>Example</title>
        <style>
            canvas {border: thin solid black}
            body > * {float:left;}
        </style>
    </head>
    <body>
        <canvas id="canvas" width="500" height="140">
            Your browser doesn't support the <code>canvas</code> element
        </canvas>
        <script>
            var canvasElem = document.getElementById("canvas");
            var ctx = canvasElem.getContext("2d");

            var startPoint = [50, 100];
            var endPoint = [400, 100];
            var cp1 = [250, 50];
            var cp2 = [350, 50];
            
            canvasElem.onmousemove = function(e) {
                if (e.shiftKey) {
                    cp1 = [e.clientX, e.clientY];
                } else if (e.ctrlKey) {
                    cp2 = [e.clientX, e.clientY];
                }
                ctx.clearRect(0, 0, 500, 140);
                draw();
            }
            
            draw();

            function draw() {
                ctx.lineWidth = 3;
                ctx.strokeStyle = "black";                  
                ctx.beginPath();
                ctx.moveTo(startPoint[0], startPoint[1]);
                ctx.bezierCurveTo(cp1[0], cp1[1], cp2[0], cp2[1], 
                    endPoint[0], endPoint[1]);
                ctx.stroke();
    
                ctx.lineWidth = 1;
                ctx.strokeStyle = "red";            
                var points = [startPoint, endPoint, cp1, cp2];
                for (var i = 0; i < points.length; i++) {
                    drawPoint(points[i]);    
                }
                drawLine(startPoint, cp1);
                drawLine(endPoint, cp2);
            }

            function drawPoint(point) {
                ctx.beginPath();

                ctx.strokeRect(point[0] -2, point[1] -2, 4, 4);
            }
            
            function drawLine(from, to) {
                ctx.beginPath();
                ctx.moveTo(from[0], from[1]);
                ctx.lineTo(to[0], to[1]);
                ctx.stroke();
            }
        </script>
    </body>
</html>
